Electric power cables with extruded insulation over conductors in conduits are used for distribution cables, 5 KV through 35 KV, and for transmission cables, 69 KV through 500 KV. Moisture in the form of water vapor and air enters these cable and conduit systems through the ends of the conduit and by diffusion through the conduit walls. Moisture causes "water treeing" in extruded insulation in a few months, reducing the life of the insulation to approximately one fourth of an expected 30 to 50 year life. Oxygen ages extruded insulation.
Moisture barriers, such as lead or aluminum sheaths, cemented metallic tapes, have been used over extruded insulation in newer installations to prolong their life, but add to the cost of the cable and do not exclude oxygen. Moisture and oxygen can corrode, or otherwise damage, aluminum and aluminum-aluminum and aluminum-copper connections. At a higher cost, extruded insulation compounded with chemical additives has been developed to prolong insulation life in wet locations. Because sodium reacts violently in contact with water, sodium conductors are not practical in the presence of moisture. Should an arc occur in a cable, the presence of oxygen makes the damage due to burning or explosion much greater. The problems caused by moisture and oxygen can be eliminated by evacuating the cable and conduit system to remove all moisture and air, then pressurizing it with an inert gas.
Unpressurized extruded insulated cables in use during recent 10 years were designed with extruded semiconducting materials under and over the insulation to exclude air at the surfaces of the insulation and to prevent ionization at operating voltage. Insulation voids, impurities, non-uniform conducting surfaces, curing ingredients, migration of materials in contact, and mixing ingredients have been a source of ionization which shortens the life of the cable. More recently, very special and expensive manufacturing methods have been developed for extruded insulated cables such as dry curing, gas curing, bonding of semiconducting materials to the insulation by triple extrusion, and special filtering of materials to remove impurities. All of these problems can be eliminated by pressurizing the insulation, conductor, shield, ground, or neutral with a gas at a lower cost.
For energy conservation, very large conductor cables are required which are economical. The present design is a compact segmental conductor or annular conductor, both of which have low skin effect and alternating current losses, but they are very expensive and limited in size. A gas pressurized cable permits the use of bundles of aluminum redraw rods or sodium rods which have little, if any skin effect, and low alternating current resistance.
Several types of gas pressurized cable and conduit systems have been used, one uses an expensive lead sheath and is pulled into a conduit in the field and then sealed. The second type is shown by U.S. Pat. No. 4,343,665 by Pugh, issued Aug. 10, 1982, and uses machine wrapped crepe paper and a gas for insulation with a factory applied conduit.
A gas such as nitrogen or sodium hexaflouride (SF.sub.6) has been used to pressurize cable and conduit systems. The voltage required to ionize a gas is proportional to the pressure and the density of the gas and its chemical makeup. SF.sub.6 is electronegative gas and is five times the density of nitrogen. The ionization voltage is three to five times that of nitrogen for the same gas pressure. Typically the gas used would be SF.sub.6 at a pressure of 20-80 pisg. By using SF.sub.6 instead of nitrogen, a lower pressure can be used for the same voltage rating. When the gas in a conduit ionizes it becomes a conductor which can act as a shield and a path for ground fault current which causes the circuit breaker to open the circuit which reduces damage to the conductors and does not damage the insulating properties of the gas.
The life of conventional ("conventional" as used herein means--used before Jan. 1, 1985) cable and conduit systems with extruded insulation and no vapor barrier that have been installed can be lengthened by fabricating the terminations and pressurizing with gas at the site.
Damage to conduits with pressurized gas can be reduced by adding an insect and rodent repellent as the gas that is diffused through the conduit wall helps keep them away.
By pressurizing a conduit with gas, damage to the cable can be reduced or eliminated if a digger pierces the conduit, hears the noise of escaping gas and stops before greater damage can occur.
A leak of gas in the system will cause a pressure drop along the line due to gas flow. By measuring the pressure along the line at splices on both sides of the leak, the location of the leak can be determined.